Ink jet printing processes are mainly of two types: continuous stream and non-continuous stream.
In continuous stream ink jet printing, a continuous ink stream is emitted under pressure through a nozzle. The stream breaks up into droplets at a certain distance from the nozzle. If a specific location on the recording sheet has to be printed the individual droplets are directed to the recording sheet, otherwise they are directed to a collecting vessel. This is done for example by charging unnecessary droplets in accordance with digital data signals and passing them through a static electric field which adjusts the trajectory of these droplets in order to direct them to the collecting vessel. The inverse procedure may also be used wherein uncharged droplets are collected in the collecting vessel.
In the non-continuous process, or the so-called “drop-on-demand” process, a droplet is generated and expelled from the nozzle in accordance with digital data signals only in the case where a specific location on the recording sheet has to be printed.
The printing speed of modern ink jet printers is ever increasing for economic reasons. Recording sheets suitable for these printers therefore need to absorb the inks very quickly. Especially suitable are recording sheets containing nanocrystalline, nanoporous inorganic compounds, preferably oxides such as aluminum oxides or silicon dioxide, or oxide/hydroxides such as aluminum oxide/hydroxides. Such recording sheets are known as “nanoporous” recording sheets. For economic reasons, nanoporous recording sheets containing fumed silicon dioxide with a positively charged surface as nanoporous inorganic oxide are particularly preferred.
Nanoporous recording sheets absorb the inks very rapidly (in the microsecond range) by the action of the capillary forces of the nanoporous compounds. Polymer based recording sheets absorb the inks more slowly (in the millisecond range) by swelling of the polymer. Recording sheets on porous paper supports absorb the inks at comparable speeds by swelling of the paper felt.
For economic reasons, it would be desirable that also in the case of high, but not highest quality ink jet printing, cheaper paper supports than paper supports with a polyolefin layer on both sides could be used without a significant quality loss. Such recording sheets would have a slightly lower gloss than the corresponding recording sheets on paper supports with a polyolefin layer on both sides, but they would nevertheless have suitable properties in many fields of application.
Patent application DE 10,020,346 describes a recording sheet for ink jet printing, which contains silicon dioxide with a size of the primary particles of at most 20 nm prepared in the gas phase, wherein the surface of the silicon dioxide has been positively charged by a treatment with polyaluminum hydroxychloride.
Patent application WO 00/20,221 describes a recording sheet for ink jet printing, wherein the ink-receiving layer contains silicon dioxide prepared in the gas phase and wherein the surface of the silicon dioxide has been modified by a treatment with aluminum chlorohydrate. This surface is positively charged.
Patent application WO 02/094,573 describes a recording sheet for ink jet printing, wherein the ink-receiving layer contains silicon dioxide prepared in the gas phase and wherein the surface of the silicon dioxide has been modified by a treatment with aminoorganosilanes. This surface is also positively charged.
Patent application EP 1,655,348 describes a recording sheet for ink jet printing, wherein the ink-receiving layer contains silicon dioxide prepared in the gas phase and wherein the surface of the silicon dioxide has been modified by a treatment with the reaction products of a compound of trivalent aluminum with at least one aminoorganosilane. This surface is also positively charged.